This invention relates generally to radiotelephone systems and, in particular, to a radiotelephone system having third generation wideband code division multiple access (WCDMA) capability.
A proposed IS-95 third generation (IS-95 3G) radiotelephone system has a wideband, spread spectrum radio interface that uses CDMA technology. The system is expected to meet all of the requirements for the next generation evolution of the current TIA/EIA-95-B family of standards. This includes providing support for the following: a wide range of operating environments (indoor, low mobility, full mobility, and fixed wireless); a wide performance range (from voice and low speed data to very high speed packet and circuit data services); and a wide range of advanced services (including voice only, simultaneous voice and data, data only, and location services). Support is also provided for an advanced Multimedia Quality of Service (QoS) Control capability supporting multiple concurrent voice, high speed packet data, and high speed circuit data services, along with sophisticated QoS management capabilities. A modular structure is proposed to support existing Upper Layer Signaling protocols as well as a wide range of future third generation Upper Layer Signaling protocols. The proposed system is also expected to provide a seamless interoperability and handoff with existing TIA/EIA-95-B systems, and to provide a smooth evolution from existing TIA/EIA-95-B based systems (including support for overlay configurations within the same physical channel as existing TIA/EIA-95-B systems). The proposed system will also support highly optimized and efficient deployments in clear spectrum (in cellular, PCS, and IMT-2000 spectrums), and will offer support for existing TIA/EIA-95-B services, including speech coders, packet data services, circuit data services, facsimile services, Short Messaging Services (SMS), and Over the Air Activation and Provisioning.
In a system operating according to the TIA/EIA-95-B standard a mobile station provides three techniques for output power adjustment (see Section 6.1.2 of IS-95). The three techniques are an open loop estimation based solely on mobile station operation, a closed loop correction that involves both the mobile station and the base station, and an outer loop Frame Error Rate (FER) based technique. In the closed loop and the outer loop correction techniques, the mobile station responds to power control bits received over a forward traffic channel to adjust its output power level.
Power control in a CDMA system is also described in a publication entitled an xe2x80x9cIntroduction to CDMA and the Proposed Common Air Interface Specification (CAI) for a Spread Spectrum Digital Cellular Standardxe2x80x94An Overview of the Application of Code Division Multiple Access (CDMA) to Digital Cellular Systems and Personal Cellular Networksxe2x80x9d, by QUALCOMM Incorporated, Mar. 28, 1992. As is described in this publication, the goal of the mobile station transmitter power control process is to produce, at a cell site receiver or base station, a nominal received signal power from each mobile station transmitter that is operating within the cell. If all mobile stations are so controlled, the end result is that the total signal power received at the cell site from all the mobile stations is equal to the nominal received power times the number of mobile stations. It can therefore be appreciated that the control of the transmitter power is an important consideration when designing mobile stations for operation in the CDMA telecommunications systems.
Of particular interest to the teaching of this invention is the closed loop power control in the forward link transmissions from a base station to a mobile station in the proposed IS-95 3G radiotelephone system. In the IS-95 3G system, power control on the forward link is performed every 1.25 ms or at an 800 Hz refresh rate. As such, a mobile station may request more power or less power for its traffic channels and the power control on the forward link occurs in the base station.
In general, mobile stations employ power control algorithms to determine the power levels required for effective operation. Typically, power control algorithms require that estimates of a traffic channel""s signal to noise ratio (SNR) are performed in the mobile station. The SNR and other factors are utilized by the power control algorithm to determine an appropriate power level for effective mobile station operation.
As is known by those skilled in the art, link performance is better with power control for a mobile station moving at a low velocity than for a mobile station moving at a high velocity. These performance observations are presented in a paper entitled xe2x80x9cThe Evolution of IS-95 to a Third Generation System and to the IMT-2000 Eraxe2x80x9d, by Edward G. Tiedemann, Jr., Yu-Cheun Jou, and Joseph P. Odenwalder, ACTS Mobile Communications Summit ""97, Vol. 2, pages 924-929, dated Oct. 7-10, 1997.
In FIG. 3 of the paper by Tiedemann et al. (reproduced herein as FIG. 3C) the traffic channel Ec/Ior (dB) to achieve a 1% Frame Error Rate is plotted versus mobile station velocity. As is illustrated in FIG. 3C, link performance is better with power control for a mobile station travelling at a low velocity than for a mobile station travelling at a high velocity, as compared to the case where there is no power control. A mobile station travelling at a high velocity is generally experiencing shifts in a carrier frequency due to the relative motion between the mobile station and the base station. This shifting in frequency is well known to those skilled in the art as the Doppler effect of wave propagation between non-stationary points. As a result, a mobile station travelling at a low velocity can be referred to as a mobile station in a low Doppler condition, and a mobile station travelling at a high velocity can be referred to as a mobile station in a high Doppler condition.
There are several possible reasons for the degradation of link performance in mobile stations moving at a high velocity (i.e. in a high Doppler condition), for example, a velocity greater than about 30 km/h for a carrier frequency of 2 GHz in a PCS band (actually about 1.86 GHz). These possible reasons include, for example, the fact that the channel is changing too fast for the mobile station to accurately estimate the channel response, and the fact that delays occur within the closed loop power control process. The delays in the closed loop power control process may be due to delays between the channel measurement performed at the mobile station and the actual change in power at the base station. Other delays may be experienced at the base station as the base station extracts and processes information from the channel. For example, the mobile station performs processing operations to determine whether or not the mobile station requires more or less power and then transmits a power control command on the reverse link to the base station. The base station decodes the command received over the reverse link and then applies the power control command to change the power of the traffic channel.
The gain due to closed loop power control, expressed as the ratio of the energy of an information bit (Eb) to the noise power spectrum density (Nt) or Eb/Nt, can be as large as 2-6 dB for low Doppler conditions (e.g., less than 50 Hz in the PCS band (2 GHz)) for mobile stations moving at a velocity of less than about 30 km/h, and as much as about 60 km/h in the cellular band (1 GHz, actually about 800-900 MHz). In high Doppler conditions (e.g., in the PCS band for mobile stations moving at a velocity above about 30 km/h, and over about 60 km/h in the cellular band), the closed loop power control results in degraded Eb/Nt performance, with the degradation being as large as 1-2 dB.
It can thus be appreciated that it would be desirable to have a power control technique which combats the effects of fading in both the low and the high Doppler conditions, and which enables mobile stations to operate at power levels which are appropriate given the velocity of the mobile station.
It is thus a first object and advantage of this invention to provide a power control technique that overcomes the foregoing and other problems.
It is another object and advantage of this invention to provide a wireless telecommunications system employing a power control technique which, in response to a determined velocity of the mobile station, adjusts the power level of a transmitted communication channel.
It is a further object and advantage of this invention to provide an improved third generation, spread spectrum wireless telecommunications system employing a power control technique which, in response to a determined velocity of the mobile station, adjusts the power level of a transmitted communication channel.
Further objects and advantages of this invention will become more apparent from a consideration of the drawings and ensuing description.
The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
In a wireless telecommunications mobile station, which is bi-directionally coupled to a base station through a communication channel, the mobile station includes a device for receiving signals over the communication channel from the base station, power control circuitry for selectively deriving power control commands, and a device for transmitting the selectively derived power control commands to the base station. The power control circuitry provides, in cooperation with the base station, closed loop power control to the communication channel. The power control circuitry includes circuitry for selectively deriving power control commands based on one of first power control commands or second power control commands in response to a determined mobility of the mobile station relative to the base station. The determined mobility may be based on a measured Doppler shift in a received signal from the base station, such as a shift in a carrier frequency, and/or an estimated or inferred velocity that is derived from the Doppler shift. Also included is a device for selectively deriving the first power control commands from the communication channel and for deriving the second power control commands from one of default power control commands and modifications to the first power control commands. The first power control commands are used when the Doppler shift or the inferred velocity of the mobile station is less than a predetermined level. The second power control commands are used when the Doppler shift or the inferred velocity of the mobile station is greater than or equal to the predetermined level. In one embodiment, the predetermined level is a velocity threshold value of above about 30 km/h when operating in a frequency band of about 2 GHz, and over about 60 km/h when operating in a frequency band of about 1 GHz.
A method of operating a wireless telecommunications mobile station is also presented. The method includes steps of: (a) providing at least one base station bi-directionally coupled to the mobile station; (b) transmitting a communication channel between the base station and the mobile station; (c) determining a Doppler condition of the mobile station; (d) in response to the determined Doppler condition, selectively deriving power control commands based on one of first power control commands or second power control commands to provide closed loop power control to the communication channel in response to the Doppler condition of the mobile station; and (e) transmitting the selectively derived power control commands to the base station.
A wireless telecommunications base station is also presented. The base station is bi-directionally coupled to at least one mobile station through a communication channel. The base station includes a device for receiving power control signals over the communication channel from the mobile station, and power control circuitry for providing, in cooperation with the mobile station, closed loop power control to the communication channel. The power control circuitry includes circuitry for selectively deriving power control commands based on one of first power control commands or second power control commands in response to a velocity of the mobile station. In one embodiment, the circuitry for selectively deriving power control commands includes a device for estimating a Doppler condition of the mobile station, a device for inferring the velocity of the mobile station, and a device for selectively deriving the first power control commands from the received power control signals and for deriving the second power control commands from one of default power control commands and modifications to the first power control commands. The device for inferring the velocity may be responsive to at least one of a speed estimation algorithm, a signal strength measurement, or a received signal error rate -measurement, such as a bit error rate (BER), symbol error rate (SER), or frame error rate (FER).
Also presented is a method of operating a wireless telecommunications system. The method includes steps of: (a) providing at least one mobile station bi-directionally coupled to a base station; (b) transmitting a communication channel between the base station and the mobile station; (c) determining a Doppler condition of the mobile station; and (d) in response to the determined Doppler condition, selectively deriving power control commands based on one of first power control commands or second power control commands to provide closed loop power control to the communication channel in response to the Doppler condition of the mobile station.